I like to use infinite scrollback. Since recently, it takes several seconds to switch to tabs with many thousands of lines of scrollback, such as from compiling a large project with qmake. From the attached backtrace (and other gdb info dump) it looks like konsole's filtering for highlighting etc. is what is taking so long. I think it's probably scanning more than it has to, which would be really just the current screen. And it probably handled it more cleverly before.
Needless to say, the current behavior is quite annoying.

Reproducible: Always

Steps to Reproduce:
1. Get a few thousand lines of scrollback in a tab (compiling something with qmake works well)
2. Make sure that the scrollback is still growing (konsole is smart enough not to rescan completely constant contents)
3. Switch to another tab
4. Switch back to the tab with lots of lines

Actual Results:  
konsole hangs for some time before the tab with the long scrollback is shown.

Expected Results:  
konsole shows the tab with the long scrollback quasi-instantly, which it used to do before.

I think this is major because it kind of kills the infinite scrollback feature and even makes tab switching with "just" 1000 lines a bit slower.